1. Field of the Invention
This invention relates to thermal ink jet printing devices and, more particularly, to thermal ink jet printheads having bubble generating heating elements or transducers with improved performance.
2. Description of the Prior Art
Though thermal ink jet printing may be either a continuous stream type or a drop-on-demand type, its most common type is that of drop-on-demand. As a drop-on-demand type device, it uses thermal energy to produce a vapor bubble in an ink-filled channel to expel a droplet. A thermal energy generator or heating element, usually a resistor, is located in the channels near the nozzle a predetermined distance therefrom. The resistors are individually addressed with a current pulse to momentarily vaporize the ink and form a bubble which expels an ink droplet. As the bubble grows, the ink bulges from the nozzle and is contained by the surface tension of the ink as a meniscus. As the bubble begins to collapse, the ink still in the channel between the nozzle and bubble starts to move towards the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in the separating of the bulging ink as a droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity of the droplet in a substantially straight line direction towards a recording medium, such as paper.
The environment of the heating element during the droplet ejection operation consists of high temperatures, frequency related thermal stress, a large electrical field, and a significant cavitational stress. The mechanical stress, produced by the collapsing vapor bubble, in the passivation layer over the heating elements are severe enough to result in stress fracture and, in conjunction with ionic inks, erosion/corrosion attack of the passivation material. The cumulative damage and materials removal of the passivation layer and heating elements result in hot spot formation and heater failure. Accordingly, a protective layer, such as tantalum (Ta) is generally provided over the heating elements or resistors and their passivation layer to reduce the cavitational damage.
In the side shooter configuration of a thermal ink jet printhead, the flow direction of the ink to the nozzle and the trajectory of the expelled droplet are the same and this direction is parallel to the surface of the resistors. This is the printhead configuration of the present invention, though the improved heating elements of the present invention are equally helpful in the roof shooter configuration, wherein the droplets are expelled in a direction perpendicular to the heating elements from nozzles generally aligned thereover.
In prior art heating elements, there is as much as 100.degree. C. temperature difference between the temperature at the center and at the edges of a 45 to 50 micrometer wide heating element. The temperature also falls off at the ends in the longitudinal direction (i.e., along the length of the ink channel) because the heating element length in this direction is significantly longer than the active length. By active length it is meant that portion of the resistive material that is used to form the bubble and is roughly that portion underneath the exposed tantalum protective layer or pit, if a thick film layer is used as disclosed in U.S. Pat. No. 4,638,337 to Torpey et al (refer to FIG. 3). Some energy is wasted in this non-active portion of electrode interface, and this wastage may be reduced by shortening the length of the heating element in that direction. However, the problem of non-uniformity in the transverse direction remains, even for a shortened heating element. At the threshold energy input, only the center of the heating element surface reaches the nucleation temperature. The edges of the heating element are significantly at lower temperatures. The bubble formation in that situation is not strong and stable enough to produce useful ink drops. Therefore, it is necessary to increase the energy input to the heating element, so that a major portion of the heater surface exceeds the nucleation temperature, and the printhead is able to produce and expel large and fast ink droplets. Experience has shown that as much as 20% energy increase over the threshold energy is required to achieve this objective. Because of the larger energy input to the heating element, the temperature in the control region of the heating element far exceeds the nucleation temperature. Referring to FIG. 5, this energy increase is necessary to produce a large enough bubble to expel a droplet of appropriate size. Thus, the heating elements must be driven to higher temperatures than would be necessary if the transverse temperature profile were uniform. The drop size dependence on energy is probably a result of the non-uniform transverse temperature across the width of the heating element.
The ink jet industry has recognized that the operating lifetime of the ink jet printhead is directly related to the number of cycles or bubbles generated and collapsed that the heating element can endure before failure. Various approaches and heating element constructions are disclosed in the following patents, though none heretofore have solved the problem of non-uniform temperature distribution across the width of the heating element in a direction transverse to the droplet trajectory.
U.S. Pat. No. 4,725,859 to Shibata et al discloses an ink jet recording head which comprises an electro-thermal transducer having a heat generating resistance layer and a pair of electrodes connected to the layer, so that a heat generating section is provided between the electrodes. The electrodes are formed thinner in the vicinity of the heat generating section for the purpose of eliminating a thinning of the passivation layer at the corners of the step produced by the confronting edges of the electrodes adjacent the heat generating section of the resistance layer.
U.S. Pat. No. 4,567,493 and U.S. Pat. No. 4,686,544, both to Ikeda et al disclose an ink jet recording head having an electro-thermal transducer comprising a pair of electrodes connected to a resistance layer to define a heat generating region. U.S. Pat. No. 4,567,493 discloses a passivation layer 208 that prevents shorting of electrodes, and a second passivation layer 209 prevents ink penetration and enhances liquid resistivity of the electrode passivation layers. Third layer 210 protects the heat generation region against cavitational forces. U.S. Pat. No. 4,686,544 discloses a common return electrode that covers the entire surface of the substrate 206 and overlying insulative layer 207 containing the plurality of transducers with openings therein for the placement of the heat generating regions.
U.S. Pat. No. 4,339,762 to Shirato et al discloses an ink jet recording head wherein the heat generating portion of the transducer has a structure such that the degree of heat supplied is different from position to position on the heating surface for the purpose of changing the volume of the momentarily produced bubbles to achieve gradation in printed information.
U.S. Pat. No. 4,370,668 to Hara et al discloses an ink jet recording process which uses an electro-thermal transducer having a structure laminated on a substrate including a resistive layer and addressing electrodes. A signal voltage is applied to the resistive layer while a second voltage of about half the signal voltage is applied to a tantalum protective layer electrically isolated from the transducer by a passivation layer. Such an arrangement elevates the dielectric breakdown voltage and increases the recording head lifetime.
U.S. Pat. No. 4,532,530 to Hawkins discloses a thermal ink jet printhead having heating elements produced from doped polycrystalline silicon. Glass mesas thermally isolate the active portion of the heating element from the silicon supporting substrate and from electrode connecting points.